The periodic disturbance in transmittance of the polystyrene photonic crystal with a diameter of 3μm is observed in our experiment. According to the peak-to-peak intervals of the periodic disturbance, it can be seen that the periodic disturbance does not come from Mie scattering or Bragg diffraction, but from the propagation of whispering gallery mode in polystyrene spheres. The results show new potential applications of phtonic crystals.